Absolutely nothing. Google gets the concept of not messing around with GUI and just focusing on what really matters: performance and features. If only Mozilla could take a hint, they could catch up on those areas.

I just don't like them aesthetically. I would like something more minimal and (at least apparently) more OS integrated.

I disagree. The tabs in Chrome are very simple, minimalist and efficient. I really don't like the angular tabs seen in other browsers and Mozilla has clearly come to the same conclusion as they're looking to change to a style similar to Chrome's tabs. Google hit on a very good UI from the get-go and it's one of the few browsers to not undergo considerable visual changes.